1. Field of the Invention
This invention relates to a method for pictorially displaying a diffraction image in a transmission-type, scanning, corpuscular-beam microscope in which the diffraction image is moved by deflection means in line-raster fashion in a predetermined direction over beam detector means, the latter of which generates an output signal for controlling the brightness of a television monitor for displaying the diffraction image.
2. Description of the Prior Art
Methods of the foregoing type are known in the art. See, for example, German Offenlegungsschrift No. 2,302,689 and U.S. Pat. No. 3,849,647. In such prior art methods, a scanning electron beam is deflected by means of a deflection system disposed above a specimen to be examined so that the beam always penetrates the specimen in a direction which is perpendicular to the specimen. Such microscopes may include an additional deflection system. If in carrying out such a method, the additional deflection system is not excited, the objective lens of the microscope provides a diffraction image in the plane of the beam detector of the microscope which is always located at the same point regardless the specimen area which is being irradiated. The position of the diffraction image can be changed relative to the beam detector of the microscope by using the additional deflection system. The diffraction image can therefore be moved over the detector in raster fashion and can be successively recorded and displayed.
In another method for changing the position of the diffraction image relative to the beam detector of the microscope known in the prior art, the additional deflection system for deflecting the diffraction image below the objective lens of the microscope is eliminated. To permit this, the deflection system which moves the incident electron beam over the specimen in raster fashion is excited by a signal having a frequency which is higher than the raster frequency so that the electron beam of the microscope no longer penetrates the specimen only in a perpendicular direction but, instead, also penetrates the specimen in an oblique manner, i.e., at different angles with respect to the optical axis of the microscope.
The intensity of the null reflex in the diffraction image is much greater than that of the higher order reflexes of the image. This high intensity of the null reflex can easily cause overloading of the beam detector of the microscope during scanning of the detector by the diffraction image since this intensity decreases only after a recovery time has elapsed, and may even lead to a permanently-lowered detector sensitivity. As a result, a raster image which is heavily infested with noise is obtained.
In order to prevent generation of such a diffraction image, it was previously necessary in both prior art methods described herein to reduce the intensity of the microscope beam so that the null reflex would be imaged properly without overloading the beam detector of the microscope. The disadvantage of this solution to the problem of noise interference, however, is that higher-order reflexes often become so weak in intensity that they can no longer be distinguished from the background in the diffraction image.